1. Field of the Invention
The present invention relates to a vertical bright-annealing furnace for continuous heat treatment of metal strips and, more particularly, to a structure of a heating zone of a vertical bright-annealing furnace in which strips of a metal such as, for example, a stainless steel, copper or aluminum, are continuously annealed while keeping the brightness of surfaces thereof by heating the metal strips to an annealing temperature in a protective gas to prevent them from oxidation and decarbonization and then cooling the heated metal strips to a predetermined low temperature.
2. Description of the Prior Art
Conventionally, an up-heat type vertical bright-annealing furnace for annealing strips of a stainless steel is composed of (i) a heating zone which heats up an upward-moving strip to the first pre-set temperature, e.g. 1,100.degree. C.; (ii) a slow-cooling zone which gradually reduces the temperature of the strip; (iii) a second cooling zone which reduces the temperature of the strip to the second pre-set temperature, e.g. 80.degree. C.; (iv) a top-roll chamber which directs the cooled strip downward; (v) an outlet chute which directs and exits the downward-moving strip; and (vi) an inlet seal section, located at a section of the heating zone where the strip enters, and an outlet seal section located at the outlet chute section, both of which are located substantially at the same level to prevent the protective gas in the furnace from leaking out as well as to prevent the outside air from entering the furnace.
According to the structure of the heating zone, such vertical bright-annealing furnaces can be roughly classified into two types, i.e., (a) a muffle-type furnace and a refractory-type furnace. The former has a cylindrical muffle which is supported at its top and covers the entire heating zone; and burners and electric heaters heat the muffle which, in turns, heats the strip moving inside the muffle. The latter has walls made up of firebricks inside, covering the entire heating zone; and electric heaters heat the strip, moving inside the walls.
When comparing the muffle-type furnace with the refractory-type furnace, although the former has two advantages: (i) its dew-point adjustment time, i.e., a time required for adjustment of the protective gas in the furnace to the working condition, is only about twenty-four hours, and (ii) its running cost is low because of its low consumption of the protective gas, there are two disadvantages: (i) the maximum temperature in the furnace is only about 1,150.degree. C. because of the heat-resistant steel used in the muffle, and (ii) the maximum length of the heating zone, i.e., the length of the muffle, is limited because of its weight, thus making it very difficult to make the furnace larger and more productive.
On the other hand, the latter refractory-type furnace is easier to make the furnace larger and enables to obtain the same level of productivity of the muffle-type with only seventy percent of the furnace length as the maximum temperature can be raised to 1,250.degree. C. and above by use of high-heat resistant firebricks. However, it has disadvantages in that it takes about four to five days to bring up to the working condition at the beginning, and its running cost is high because of its high consumption of the protective gas.